Pulp or papermaking machines, utilize machine rolls. Such machine rolls are used during various aspects of the process in, for example, the press, forming, drying or calendering sections. The operation of machine rolls requires a device to remove contaminants which form on the roll surface and/or to pull off the sheet from them. A traditional method of achieving this is through the use of a mechanical device commonly referred to as a doctor or doctor blade. The failure to remove the contaminants or the sheet effectively can have a catastrophic effect on the quality of the product being produced.
The doctor blade is typically fastened to a structural beam which is adjustably supported across the papermaking machine on which a blade holder and replaceable blade is provided. The doctor blade comes in direct contact with the roll surface so as to scrape off any contaminants from the roll surface including the whole pulp or paper web sheet or parts thereof. In certain applications the material is very difficult to remove. In this regard a second doctor blade in addition to the first may be desireable to provide an added scraping factor. This allows for the roll surface to be scraped twice as much as it rotates. In addition, the space defined by the two blades provides a safe location to apply a cleaning fluid on the roll surface without contaminating or disturbing the pulp or paper web nearby.
Heretofore the means of providing two blades on the same machine roll is typically that illustrated in FIGS. 1 and 2. In FIG. 1 there is shown a machine roll 1 having two separate and independent standard doctor blade devices 2 and 3. This arrangement is quite desireable since the doctor blades may be independently adjusted with respect to the roll. Furthermore, if there is sufficient space in between the two devices each would be able to scrape the contaminants, sheet or web from the roll and deflect it away, down to a broke pit or to a safe location outside the process. In addition, such a design permits easy access for cleaning and maintenance of the blades, the blade holders and support structure. However, while such an arrangement has certain desirable features, it also has a significant drawback, that being in existing papermaking machines or in the design of new machines there simply is not enough room to install a complete and independent second doctor blade unit.
Another method of incorporating a second doctor blade on the same pulp or paper machine is shown in FIG. 2. Again a standard machine roll 4 is illustrated. To save space, a second blade 7 and blade holder 8 is added trailing the first blade 5 and blade holder 6 all of which is supported by a single solid structure 9 which may be of conventional construction. The problems with this design are many. The scrapings removed by the second blade fall back and accumulate on the support structure 9 in the closed space created in between the blades. During the operation of the machine, this area can not be cleaned and will eventually not only impede the effective operation of the blades but also it is possible that the high speed web will become stuck between the two blades and force the structure to bend away from the machine roll causing immediate damage to it.
Accordingly, there exists a need to provide for a doctor blade arrangement utilizing two doctor blades which has the advantages of the foregoing arrangements yet avoids their disadvantages.